1. Field of the Invention
The present invention relates to a projection exposure apparatus, and more particularly, it relates to a projection exposure apparatus wherein alignment marks formed on a substrate such as a semiconductor wafer, a liquid crystal display element plate or the like are detected by an alignment sensor of off-axis type in order to align a mask image with such a substrate.
2. Related Background Art
In manufacturing semiconductor elements, liquid crystal elements or the like by means of a photo-lithography process, there have been used projection exposure apparatuses wherein a pattern image of a photo-mask such as a reticle is projected by means of a projection optical system so as to expose each of shot areas on a substrate (for example, a wafer, a glass plate and the like) on which photosensitive material is coated.
The conventional projection exposure apparatus had to include an auto-focus mechanism for detecting a position (height) of the wafer along the optical axis of the projection optical system at an exposure position so as to make the detected position coincide with an imaging plane of the projection optical system. Recently, since the depth of focus is made shallower as a result of an effort to obtain a high resolution of the projection optical system, there arose a problem that the resolution in the exposure area of the wafer and the uniformity of line width of the projected image are worsened when unevenness and inclination of the wafer exist. To avoid this, the conventional projection exposure apparatus also includes a mechanism (auto-leveling mechanism) for detecting and controlling a horizontal position of the wafer at each exposure position, as well as the aforementioned auto-focus mechanism. An example of combination of these two mechanisms is disclosed in U.S. Pat. No. 4,558,949 which shows integral combination of a level detection system of oblique incident collimater type and a focus detection system of oblique incident type.
In recent years, in order to manufacture ICs having higher integration, there is a strong tendency that the wavelength of exposure light is shortened, thereby obtaining the high resolution. To this end, for example, it has been proposed that KrF excimer laser beam (wave length .lambda.=248.5 nm) is used as the exposure light. In case of a projection exposure apparatus using the KrF excimer laser beam, even when alignment between the reticle and the wafer is performed by a TTL (through the lens) system wherein alignment light having wavelength different from that of the exposure light is illuminated on the substrate through an exposure projection lens, such apparatus has no light source for emitting alignment light having appropriate wavelength near that of the exposure light. Thus, the wavelength of the exposure light greatly differs from that of the alignment light. Since the projection lens is so designed as to reduce or eliminate color aberration with respect to the particular wavelength of the exposure light, it is difficult to provide an alignment optical system in which the color aberration is properly corrected.
Further, even when the alignment is performed by using the excimer laser beam itself, there are many problems to be solved. For example, a problem that the photosensitive material such as photoresist coated on the wafer is exposed, and a problem regarding wide dispersion in output pulse from the excimer laser which itself is a pulsating light source must be solved. Therefore, in a projection exposure apparatus using a deep-ultraviolet radiation source, it is effective to provide an alignment sensor of off-axis type spaced apart from the projection optical system by a predetermined distance and adapted to exclusively detect alignment marks on the wafer, since, when the alignment sensor of off-axis type is used, there is substantially no limitation regarding the wavelength of the alignment light and the way of detection thereof, with the result that alignment having high reproductivity can be expected.
However, in the projection exposure apparatus having the conventional alignment sensor of off-axis type, the auto-focusing and auto-leveling are performed only at the exposure position (exposure field of the projection optical system). In this case, if the positional deviation of the imaging plane of the projection optical system occurs due to the fluctuation of the atmospheric pressure or the like, the best focus position of the alignment sensor of off-axis type do not change correspondingly. Therefore, even when the exposure plane of the wafer is aligned with the imaging plane of the projection optical system by means of the auto-focus system, out-of-focus occurs in the alignment sensor of off-axis type, thereby worsening the accuracy for detecting the alignment marks on the wafer.
Further, when the exposure continues for a long time, the position of the imaging plane of the projection optical system may be changed along the optical axis by the thermal deformation due to the continuous illumination of the exposure light. Also in this case, since the best focus position of the alignment sensor of off-axis type does not change, when the exposure plane of the wafer is aligned with the imaging plane of the projection optical system, out-of-focus will occur in the alignment sensor of off-axis type.
Furthermore, if the surface of the wafer is inclined, when the alignment mark is detected by the alignment sensor of off-axis type in a condition that the surface of the wafer is aligned with the imaging plane of the projection optical system at the exposure position of the projection optical system, the surface of the wafer is deviated from the focus of the alignment sensor at a detection position where the alignment sensor is provided and, therefore, the detection accuracy will be worsened. Incidentally, if the alignment mark is shifted to the exposure position of the projection optical system and then the surface of the wafer is aligned with the imaging plane of the projection optical system by the auto-focus action and then the alignment mark is shifted to the detection position where the alignment sensor is provided, the alignment mark will be brought to the best focus position and, therefore, the optimum accuracy will be obtained. In this case, however, since a stage must be shifted from the exposure position to the alignment mark position (corresponding to the position where the alignment sensor is provided), throughput will be deteriorated.
In the manufacturing process for manufacturing various devices, the exposure procedures for photo-lithography are repeated. In many cases, the projection patters are different from each other each time. The pattern image projected in this case must have a predetermined positional relation to the previous pattern image on the substrate. The predetermined positional relation between the reticle and the substrate is referred to as "alignment" and the accuracy of such positional relation is referred to as "alignment accuracy". In any cases, the imaging feature of the projection optical system such as the projection magnification (reduction ratio) and/or distortion is often adjusted in the subsequent exposure process. The adjustment of the imaging feature can be achieved by using a method for adjusting the imaging feature to a predetermined imaging feature or a method for adjusting the imaging feature in consideration of a shape of the shot area on the photosensitive substrate. In the former method, the imaging feature is adjusted each time when a predetermined time period is elapsed or each time when a predetermined number of photosensitive substrates are treated. In the latter method, the imaging feature is adjusted by measuring the shape of the shot area on each photosensitive substrate and then by adapting the imaging feature to the measured shape. However, in the latter method, the photosensitive substrate may often be expanded, contracted or distorted during the treatment processes such as heat treatment and/or ion injection. If such expansion or the like occurs, the pattern images cannot be completely overlapped through the whole area under the same reduction ratio and/or distortion. When the shape of the shot area on the photosensitive substrate is changed between lots (each lot being a treatment unit for a predetermined number of photosensitive substrates), the imaging feature may be adjusted only regarding the first photosensitive substrate in each lot.
By adjusting the imaging feature in this way, the imaging plane of the projection optical system is displaced along the optical axis direction (Z-direction). Thus, the adjusting amount of the imaging feature and the displaced amount of the imaging plane are previously considered and a correspondence table representing the correspondence between the adjusting amount and the displaced amount is previously prepared. After the adjustment, when the exposure is effected, the position of the displaced imaging plane is predicted on the basis of the correspondence table, and the photosensitive substrate is aligned with the predicted position. The reason is that, if the correct position of the imaging plane is detected by using a reference member each time when the imaging feature is adjusted, a long time will be taken and the through-put of the device will be worsened.
In order to increase an amount of information treated by micro-devices, finer patterns have been requested. However, when the finer pattern images are aligned with each other by using the conventional steppers, it has been found that the good quality rate of the devices was reduced. It is considered that the main reason is that the alignment accuracy is poor.
When the imaging feature of the projection optical system is adjusted, the imaging plane of the projection optical system is displaced along the optical axis direction to be offset from the focusing plane of the alignment system. As a result, although the depth of focus of the alignment system is great, the imaging plane becomes in a significant defocus (out of focus) condition. That is to say, the detection accuracy is greatly decreased. Thus, if the mark is detected under the defocus condition, output of the detection signal will be also decreased. Particularly, in case of a mark from which a signal is hard to be emitted, reduction in output of the detection signal is great, resulting in the reduction in detection accuracy. On the other hand, if the mark is detected under the defocus condition, the detection accuracy will be decreased due to telecentric error (inclination of the optical axis) of the alignment system. However, these problems need not be considered in the conventional techniques. Strictly speaking, the alignment is incorrect more or less, and, thus, the alignment accuracy is low. However, in the past, since the patterns were not so fine, even when the alignment accuracy was low, the good quality rate of the devices was high. When the pattern becomes finer, the low alignment accuracy causes the quality of the devices to worsen, thereby reducing the good quality rate of the devices.
One way for solving the above-mentioned problems is to perform the focusing not only in a projection optical system but also in an alignment detecting system for a substrate. The present invention is directed to an effective method for effecting the focusing in the alignment detecting system for the substrate. However, even when the focusing in the alignment detecting system for the substrate is performed accurately, if a distance (baseline) between the exposure center of the mask pattern and the center of the substrate alignment detecting system is not measured correctly, it is not possible to determine the correct position of the exposure position on the substrate with respect to the mask. Thus, an apparatus for correctly measuring the baseline in a short time has been greatly requested.